(1) Field of the Invention
The present invention relates to a driving means which is formed by a single induction motor integrally having two stators and two rotors, or which is formed by two induction motors for driving a common load, and a method for starting the operation of such driving means.
(2) Description of the Related Art
Generally, there have been known a line-starting method and a star-delta connection method as the starting method of an induction motor for driving a load. The line-starting method achieves a sufficient torque but, on the other hand, a large current flows at the starting operation in the case of a large capacity motor so that the electrical installation cost is inevitably very large. In the case where the star-delta starter is used, although the current at the starting operation can be limited to be low, the starting torque is also lowered. Because the starting torque is lowered, it becomes necessary to use a larger capacity induction motor, resulting in raising the cost of the induction motor. Here, with respect to the case where a star-delta starter of a general kind is used, the resulting torque, current and input characteristics are shown in FIGS. 9, 10 and 11.
Methods for starting an induction motor have been disclosed in, for example, Japanese Patent Application Kokai Publication Nos. Sho 51-104513 and Sho 52-54112, and a number of methods for starting the motor by appropriately switching the stator windings between the star- and delta connection are known. In these methods, the stator windings are constituted by a plurality of circuits (windings) and, during the starting operation, the starting current is controlled to be low by the switching of these windings respectively to star-connections or delta-connections.
Each of the above described methods enables the limiting of the starting current but it remains as a star-delta starting method so that, due to the reduction of the starting current, the starting torque is inevitably lowered. Thus, the device that employs the star-delta starting cannot be applied other than to such loads as a load having square-law decreasing characteristics in which a load is smaller than the starting torque in the star-connections for the star-delta starting so that, although the motor is an induction motor, its application is limited to and the motor cannot serve as a general purpose induction motor. Further, the induction motor employing the above starting methods lacks characteristics of intermediate torque which is larger than the starting torque and which enables the continuous operation.
In a prior art example wherein there are a plurality of individual stator windings provided on a single stator-core, when power is supplied only to one of these stator windings, the deterioration in magnetomotive force waves is caused, resulting in various defects such as fluctuations of torque characteristics, starting failure, and lowered operation efficiency, and the resulting structure cannot be used in practice.
The induction motor that is started by the line-starting method is used mostly for a load in which a load is very small or an inertia is large, and thus it embodies a method that is dictated by the starting torque of the induction motor. However, the period in which the load having a large inertia requires a large torque is up to the time when the rotation speed reaches its rated speed. Once the rated rotation speed is reached, the rotation speed is unlikely to change especially because of the large inertia so that a smaller torque is sufficient after the starting operation. However, despite the fact that only the small torque is needed subsequent to the starting, the output cannot be lowered after the starting only because the induction motor is of the line-starting type and, under the existing state, unnecessary power is being wasted. Thus, there is a demand for the development of a driving device by an induction motor and a method for starting the same in which the starting current is reduced during the starting operation and yet to generate a large torque, and which enables the power-saved operation during the rated operation. To meet this demand, a starting compensator operation (condorfer starting) and, recently, an inverter starting operation are available, but each of these involves a device with a very large cost.
The induction motor is often used by being connected to an emergency electric generating installation. The induction motor is then used as a motor for driving an emergency fire extinguishing installation such as a pump. As to a prime mover constituting a private power generating installation for emergency purposes and a generator driven by the prime mover, the size of the prime mover and the capacity of the generator are selected based on a coefficient which is calculated on a load current change rate and an input change rate with respect to the motor rating during the starting operation period from the time when the motor which becomes the load of the electric generating installation starts to the time when the rated operation speed is reached. Therefore, as a matter of course, a study has been made not only for reducing starting current during the starting operation but also for reducing the rates of load current and input variations in the star-delta switching during the starting operation. This is because, by the reduction of the required capacity of the generator and the prime mover, it is possible to reduce the installation cost.
Also, the fire extinguishing installation such as a pump which is operated in an emergency is one in which the time required for reaching the rated operation is made as short as possible. To this end, it is desired that an induction motor as a driving apparatus be developed in which, while the load current change rate and the input change rate are given due consideration, the starting torque is made large and the starting operation period is made short to the fullest extent, and yet the structure of the apparatus be made simpler as possible. The starting methods with which the attempt has been made to realize the desire includes a closed star-delta starting, a reactor starting, and a condorfer starting including a special type of condorfer starting.
Where the induction motor is used in, for example, a compressor in which the operation is frequently started and stopped or undergoes frequent changes between high and low loads, the induction motor repeats frequent stopping and re-starting operations and, due to the starting current during the re-starting operation, the induction motor suffers from an intense rise of temperature thus causing the motor to become inoperable. In a large induction motor, a star-delta starting operation has been used in order to limit the starting current during the starting period, but the star-starting frequently fails the re-starting because of the low starting torque, and this requires the use of an induction motor of a higher ranking in its capacity. Also, for driving a compressor, an unload type motor is available in which the motor is constantly rotating and unnecessary air is appropriately being exhausted but, since the motor is constantly rotated, the running cost becomes high.
For the above reasons, each of the conventional induction motors as described above has been such that unnecessary power is consumed because of a large inertia or that, although the motor is for general purposes, a special starting device is required in order to limit the starting current, and this has rendered the motor to be that for special use and no longer that for general purposes. Thus, there has been a demand for the development of technology enabling the provision of an induction motor as a driving apparatus which is low in cost and which, as a motor that can cope with a large inertia without requiring any such special high cost starting device as required in general purpose motor, as a motor that can be used in a private power generating installation, and as a motor that can be used in a compressor that repeats starting and stopping operations and is differently used depending on a loaded state and an unloaded state, can be used efficiently for any purposes while fully exhibiting its capability as an induction motor.
The selection of an output of an ordinary general purpose induction motor which is constituted by a single rotor and a single stator and which has characteristics of the starting torque being smaller than those of the rated torque, is not based on the rated torque as reference but is based on the starting torque of the induction motor with reference to the load torque and, especially when the motor is star-delta started, the starting torque inevitably becomes small thereby requiring a kind of motor whose output is of a higher ranking, that is, a higher capacity. This requirement is more conspicuous when the load during the starting operation is high. Thus, there is a demand for techniques for developing an induction motor as a driving apparatus and a method for starting the operation of the same, which is constituted by a general purpose induction motor but which can be used in connection with a variety of loads without the need of selecting a kind of induction motor in which its output is unnecessarily ranked up.